1. Field of the Invention
The present invention relates generally to laser radar and, more particularly, to an optical backscatter filter for use with continuous wave (CW) laser radar systems.
2. Discussion
Laser radar systems, employing an intensely focused beam of light to detect the presence, position and motion of objects, have been used in many applications, especially in the radar communications and measurement fields. Militarily, these systems have been implemented in conjunction with new cruise missile and tactical fighter technology wherein laser radar has provided obstacle avoidance as well as terrain following functions. Laser radar systems have also enabled sophisticated target homing capabilities for accurately guiding a missile or plane toward a target by using a distinguishing feature of that target.
FM "chirped" laser radar which involves heterodyne or coherent detection has proven to be particularly useful in these applications. Typically in these systems, a continuous wave (CW) transmitter emits laser light at a preselected center frequency. This emitted light is frequency modulated into linear "chirps" by passing it through an electro-optical device disposed within the cavity of the transmitter. The frequency variation created is preferably linear and the frequency versus time characteristic of the signal typically has a sawtooth trapezoid pattern.
The "chirped" signal is directed toward a target and then reflected back therefrom, creating a "return" signal associated with that target. The time taken by the transmitted signal to reach the target and then return causes the return signal to be displaced in time with respect to the transmitted signal. This is shown graphically in FIG. 1 wherein the solid line represents the transmitted signal and the dashed line shows a corresponding return signal.
The instantaneous frequency difference between these signals is indicated in FIG. 1 as f.sub.o. To obtain this frequency difference, the return signal is compared to a reference signal which is typically a sample of the transmitted signal created by retaining a small portion of the transmitted beam using a beamsplitter. Properly scaled, this instantaneous frequency difference can be used to "demodulate" the return signal in order to obtain information about the target.
In practice, however, especially in monostatic or common aperture laser radar systems, the transmitted signal travels through the same optics as does the return signal. Due to imperfections in the optics, part of the transmitted signal is reflected by the optics back toward the receiving photodetector as "optical backscatter". Therefore, the received signal incident on the photodetector includes not only the return signal but also interfering transmitted light or optical backscatter. This backscatter in a monostatic or common aperture laser radar system can typically be five to eight orders of magnitude greater than the energy reflected from the radar target, thereby severely reducing the dynamic range and performance in the receiving electronics as well as producing interfering harmonics in the receiver. This optical backscatter signal is a severe problem in modulated CW laser systems wherein the receiver and transmitter are required to operate simultaneously.
However, it is possible to discriminate between the return signal and the interfering backscatter by using the frequency and time characteristics of these signals. The backscattered optical energy, being transmitted light, is substantially frequency coherent with the reference signal whereas the frequency of the return signal is displaced in time. By removing components of the received signal that are frequency coherent with the transmitted signal, i.e., the backscatter, the desired return signal can be extracted.
Conventionally, electronic backscatter filters have been employed to remove this interference from the desired return signal. The return signal and backscatter are typically electronically mixed with an electronically frequency shifted transmitted reference signal. This process usually involves very wideband signals and the electronic mixing process generates spurious backscatter signals that compete in signal strength with the return signal by generating ghost targets. These ghost targets are backscatter signals which have been generated by the electronic mixing process and which are no longer at the same frequency as the optical backscatter signal that is processed through the electronic mixer. Therefore, these ghost targets can not be eliminated by the backscatter filter because they can no longer be discriminated from real return signals.
In addition, chirp nonlinearites often result from the traditional electronic mixing process. These nonlinearites limit the amount of backscatter that can be processed as well as the bandwidth-time capability of the system. The electronic optical backscatter filter has often been labelled as a severe limitation to the produceability of FM laser radar.
For these reasons, as well as the need for unerring resolution and accuracy in critical military applications, there exists a need for an optical backscatter filter which is accurate, efficient, and reliable. Also, due to the nature of these applications wherein multiple laser radar systems requiring multiple backscatter filters are employed on one missile or plane, it is desirable that such filters be compact, simple, and less expensive than previous systems.